1. Field of the Invention
The present invention relates to an instrument power controller and method, and more particularly, to an instrument power controller and method for adaptively providing an output voltage VO and an output current IO that together maintain a substantially constant electrical output power PO.
2. Statement of the Problem
Flow meters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing materials can comprise liquids, gases, combined liquids and gases, solids suspended in liquids, liquids including gases and suspended solids, etc. A flow meter can be used to measure a flow rate (i.e., by measuring a mass flow through the flow meter), and can further be used to determine the relative proportions of components in a flow stream.
In many process control or industrial automation settings, a bus loop or instrumentation bus is used to connect to various types of devices, such as flow meters, for example. The bus loop is commonly used to deliver electrical power to the various attached instruments or devices. In addition, the bus loop is also commonly used to communicate data both to and from the sensor or device. Therefore, the bus loop is connected to a master device that can provide regulated electrical voltage over the bus and that can exchange communications over the bus. The master device can send commands and/or programming, data, calibrations and other settings, etc., to the various connected devices. The master device can also receive data from the connected devices, including identification data, calibration data, measurement data, operational data, etc.
The master device can further comprise a power supply that is connected to an electrical power source. The master device typically provides electrical power over the bus loop that is current limited, voltage limited, and power limited.
During normal operation of a vibratory flow meter, such as a densimeter or Coriolis flow meter, the current consumption and voltage requirements are relatively stable. However, when the flow meter is initially powered up, vibration of the meter flow tubes gradually increases in frequency and amplitude. Due to the construction and material of the flow tubes and due to the added mass of flow material in the flow tubes, the flow tubes cannot be immediately brought up to a target vibrational amplitude. Consequently, the startup phase will require electrical current above that required for normal operation. Therefore, the electrical current draw at startup is higher than a current draw during normal operation.
A bus loop can comprise a 4-20 milliamp (mA) bus loop, for example. The 4-20 mA bus is a two-wire instrumentation bus standard that is typically used to connect to a single instrument and is further capable of being used to provide communications between an instrument and a host device. Alternatively, the bus loop can comprise other bus protocols or standards.
According to requirements of Intrinsic Safety protection methods, the electrical power delivered by the master device/power supply is strictly limited for purposes of safety. For example, a 4-20 mA bus protocol can be limited to 20 mA of electrical current and can further be limited to 16-32 volts (V). The electrical power available to a device on the bus is therefore limited.
In some operating environments, flow tube startup can be problematic. One result of power limitation at startup time is that flow tube startup time is greatly extended, as excess current is not available for boosting the vibrational amplitude of the flow tube or tubes.